The present invention pertains to sensors which are useful for determining and controlling levels of liquid or powdered solids in containers. The measurement of level is a basic requirement in a widely varied group of industries; included, for example, are those which store or process water, oil, milk, grain, paint, dusts, powdered chemicals and cement. To serve the needs of these industries a variety of apparatus has been developed. A brief description will serve to outline prior art as it effects the present invention.
The simplest device used for liquids is the float gauge. In the latter a buoyant solid floats on top of the liquid contained in a tank and moves up and down with changes in level. The position of the float may be transmitted to the outside of the tank by means of a rope and pulley arrangement. The rope is attached at one end to the float and terminates at its other end in a moveable target. A stationary measuring scale adjacent to the target permits the observer to judge the level of liquid from outside the tank. If the tank is sealed, the level may be telemetered by placing a magnet inside the float and arranging the latter so that it must move past a series of magnetically responsive switches. As the float rises and falls, signals are obtained which can be used to actuate indicating devices positioned at remote points.
Another elementary device for liquid level indication is the sight gauge. A transparent tube is sealed into holes in the side of the container so as to constitute a vertical side arm. The tube is in hydraulic communication with both the liquid and the space above it; the liquid now fills the transparent tube to a level equal to that in the container. The level may be read directly or it may be converted to an equivalent signal by photoelectric or other means to permit telemetering and control.
Other methods for liquid level include: the measurement of pressure at the bottom of the container; the bubbling of air or other gas through a tube extending down into the liquid while measuring the pressure of the gas; the timing of ultrasonic pulses which are directed at and reflected from the interface between gas and liquid; the attenuation of gamma rays which pass down through the liquid; determining variations in weight of the entire container.
The measurement of level in vessels which hold powdered solids is somewhat more difficult and fewer methods are available. The timing of ultrasonic pulses directed from above has been successful in some cases. Another method involves the use of a plumb bob coupled to a motor and switch. The motor lowers the bob until the latter encounters solids and a slack is introduced in the bob's line. This triggers the switch and reverses the motor which now winds the bob up to its original position. The amount of line paid out is a measure of level. Another scheme for solids level sensing uses a horizontally positioned, vibrating tube which stops vibrating when it becomes immersed in the solids. The electric power fed to the vibrator coils changes sharply at this point; this change can be used to indicate that the level has reached or exceeded the position of the sensor. Because this method provides only a single point signal, it is necessary to use several sensors if intermediate level indications are required. A somewhat similar method employs a motor driven paddle mounted at some position in the vessel. When the paddle encounters solids, paddle torque rises sharply which results in a reactive torque on the flexibly-mounted motor. This actuates a switch to provide a single point signal. The drive motor continues to operate with the paddle immersed in the solids unless excessive torque is encountered in which case a second switch shuts off the motor.
Although each of the above described methods will operate well in situations to which it is suited, there are a growing number of applications for which none is totally satisfactory. In liquid level indications requiring high resolution, for example, some float devices are not suitable because of immersion depth uncertainty and because of friction in the various linkages required to operate the readout system. In the determination of liquid level interfaces in a situation where one liquid is floating on a second, floats and pressure measurement systems are not suitable because the liquids often have only a slight difference in density. Transparent tube indicators are sometimes unsatisfactory because the vessels contain dark liquids which stain the tubes and impede viewing.
Solids present special problems; present level indicators require several sensors for point to point measurement; this increases installacost and complexity. Sonic and gamma ray methods are often not accurate because of ray scattering by individual particles.
It is an object of the present invention to provide an apparatus suitable for determining the level of liquids in tanks and for measuring the interface between two immiscible liquids stored in a container. It is a further object of this invention to provide an apparatus which gives continuous level indication of powders, granulated solids and other particulates.